The easiest way would be to be born in Australia to American parents, or to be born in the USA to Australian parents. Or to be born anywhere in the world to one American and one Australian parent. I think the USA does allow citizens to hold other citizenships. Other countries may not (but Australia does). It used to be more strict.

Australian citizen by birth. Whether you are an Australian citizen by birth depends on the date of your birth. Most children born in Australia before 20 August 1986 are Australian citizens by birth unless one parent was entitled to diplomatic privileges or was a consular officer of another country.

Until 1975 Papua was a territory of Australia and Papuans held a form of Australian citizenship (although they still required a permit to travel to mainland Australia). New Guinea was a United Nations Trust Territory administered by Australia and indigenous New Guineans were classed as 'Australian Protected Persons', or APPs.

Australian Citizenship by Descent. In certain circumstances a person born who is born outside of Australia may become an Australian citizen through their parents' status as an Australian citizen. At the time of the birth at least one of the child's parents must be an Australian citizen.

If you were born in Australia before 20 August 1986. your full birth certificate issued by an Australian Registry of Births, Deaths and Marriages (RBDM) will suffice to confirm your citizenship, provided your parents were not in Australia as foreign diplomats or consular officers at the time of your birth.

If it is born in Australia, it is automatically an Australian citizen because its father is Australian. If it is born elsewhere, it qualifies for Australian citizenship by descent which must be applied for but there is no question that it will be granted if the father is on the birth certificate.